Experimenting with RGBW Color Mixing
For my latest project, I wanted to use RGBW LEDs instead of just RGB ones. As they have some interesting advantages over RGB that I wanted to explore: A more accurate representation of whiteA wider color spectrum This only works with somewhat desaturated colors. But at the end of the post I have some ideas that might help to work around that. This would allow for better lighting of surfaces as well as more natural-looking colors. Why that is and how we get there is what I will try to explain below: What is color exactly?Light and color Light or visible light to be precise is a form of electromagnetic radiation with a wavelength of 400-700 nanometers. Electromagnetic radiation means that the wave got a magnetic as well as an electric field which both oscillate at the same time. Such a wave is emitted when an electron transitions from a higher to a lower state of energy, releasing the difference in the form of a photon. The length of one of the wave segments is used to identify the waves corresponding wavelength. What the wavelength got to do with spectral color Relevant for us are the colors red, green and blue(RGB anyone?) that can be found at ~700 nm, ~530 nm, and ~470 nm respectively. At those wavelengths, they are considered (near-) spectral colors. That means that they are composed of only one wavelength or a relatively narrow band of wavelengths. Which means that they can be used to efficiently mix different colors. Mixing colorsAdditive mixing When different waves of light intersect and bundle up before they reach the eye, they can be perceived as a different color that appears lighter than the colors used to mix it. A light source will appear as white if enough colors are added up that way. And as an example for mixing, if you get a red and a green light to combine, the resulting light will appear yellow. Subtractive mixing This type of color mixing determines how an object looks in a specific light. The surface of an object is composed of one or many pigments. A pigment only reflects a specific wavelength and absorbs the rest. Mixing enough pigments will result in all light being absorbed, creating a black surface. For example, a red surface illuminated by white light looks red because it absorbs all but the red light. But if you would use a blue light instead, the same surface would look black. Wavelength spectra of RGB and white LEDsRGB LED Spectrum Let’s take a look at the emission graph above that shows what the possible spectrum of an RGB LED might look like. Given a temperature of 25 °C, the x-axis describes the wavelength and therefore the perceived color of the light and the y-axis describes the actual intensity of that wavelength. And as you can see, their actual colors are intense but have a quite narrow range of emitted colors with some gaps in between. White LED Spectrum If we now look at the spectrum of a white LED, you can clearly see that there is a wider range of wavelengths. To understand how this is archived we first need to understand what a Stokes shift is. Quoting Wikipedia: When a system (be it a molecule or atom) absorbs a photon, it gains energy and enters an excited state. One way for the system to relax is to emit a photon, thus losing its energy (another method would be the loss of energy as heat). When the emitted photon has less energy than the absorbed photon, this energy…
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